It is possible that there is a problem with files in the profile folder that store the extensions registry. Delete the extensions.* files (e.g. extensions.json, extensions.ini, extensions.sqlite) and compatibility.ini in the Firefox profile folder to reset the extensions registry. *https://support.mozilla.org/kb/Profiles New files will be created when required. See "Corrupt extension files": *http://kb.mozillazine.org/Unable_to_install_themes_or_extensions *https://support.mozilla.org/kb/Unable+to+install+add-ons If you see disabled or not compatible extensions in ""3-bar" menu button or Tools -> Add-ons -> Extensions" then check if there is a compatibility update available. *click the cogwheel button (left of the Search bar) on the about:addons page to "Check for Updates" If this didn't help then delete add-ons.json and possibly prefs.js in case there is a problem with preferences. ---- Did you possibly used the Firefox Refresh feature and created a new profile? When you use the Firefox Refresh feature then a new profile is created and some personal data (bookmarks and history, form data, tabs) is automatically imported. The current profile folder will be moved to an "Old Firefox Data" folder on the desktop. Installed extensions and other customizations (toolbars, prefs) that you have made are lost and need to be redone. It is possible to recover data from the "Old Firefox Data" folder on the desktop, but be cautious not to copy corrupted files to avoid carrying over problems.